Fix RTC backup register indexing, add target-specific register and bo…

…ot command

According to ST's RM0008 STM32F103xx reference manual section 6.4.5 table 17
the backup registers are 32 bits wide with only the lower 16 bits being used
for storing values. In addition to this, the "first" register slot is entirely
reserved and there is no backup register zero, meaning that BKP_DR1 starts from
an offset of 0x04. This was mostly correctly reflected in backup.c, but the
2-multiplier for the register enum throws everything off by a lot, which
results in the boot command check failing to work correctly when using anything
but the first backup register. BKP0 has also been removed since as described by
RM0008 indexing starts at one (BKP1 is now fully equivalent to the old BKP0).

I've also gone ahead and implemented support for specifying the backup register
and boot command to use per-target. If not defined, options equivalent to the
old behavior are used to not break existing applications.

src/stm32f103/backup.c

@@ -29,13 +29,13 @@ void backup_write(enum BackupRegister reg, uint32_t value) {
     rcc_periph_clock_enable(RCC_BKP);
 
     pwr_disable_backup_domain_write_protect();
-    RTC_BKP_DR((int)reg*2) = value & 0xFFFFUL;
-    RTC_BKP_DR((int)reg*2+1) = (value & 0xFFFF0000UL) >> 16;
+    RTC_BKP_DR((int)reg) = value & 0xFFFFUL;
+    RTC_BKP_DR((int)reg+1) = (value & 0xFFFF0000UL) >> 16;
     pwr_enable_backup_domain_write_protect();
 }
 
 uint32_t backup_read(enum BackupRegister reg) {
-    uint32_t value = ((uint32_t)RTC_BKP_DR((int)reg*2+1) << 16)
-                   | ((uint32_t)RTC_BKP_DR((int)reg*2) << 0);
+    uint32_t value = (uint32_t)RTC_BKP_DR((int)reg)
+                   | ((uint32_t)RTC_BKP_DR((int)reg+1) << 16);
     return value;
 }

src/stm32f103/backup.h

@@ -20,11 +20,16 @@
 #define BACKUP_H_INCLUDED
 
 enum BackupRegister {
-    BKP0 = 0,
     BKP1,
     BKP2,
     BKP3,
     BKP4,
+    BKP5,
+    BKP6,
+    BKP7,
+    BKP8,
+    BKP9,
+    BKP10,
 };
 
 extern void backup_write(enum BackupRegister reg, uint32_t value);

src/stm32f103/target_stm32f103.c

@@ -55,7 +55,13 @@ _Static_assert((FLASH_BASE + FLASH_SIZE_OVERRIDE >= APP_BASE_ADDRESS),
                "Incompatible flash size");
 #endif
 
-static const uint32_t CMD_BOOT = 0x544F4F42UL;
+#ifndef REG_BOOT
+#define REG_BOOT BKP1
+#endif
+
+#ifndef CMD_BOOT
+#define CMD_BOOT 0x544F4F42UL
+#endif
 
 void target_clock_setup(void) {
 #ifdef USE_HSI
@@ -160,13 +166,13 @@ const usbd_driver* target_usb_init(void) {
 bool target_get_force_bootloader(void) {
     bool force = false;
     /* Check the RTC backup register */
-    uint32_t cmd = backup_read(BKP0);
+    uint32_t cmd = backup_read(REG_BOOT);
     if (cmd == CMD_BOOT) {
         force = true;
     }
 
     /* Clear the RTC backup register */
-    backup_write(BKP0, 0);
+    backup_write(REG_BOOT, 0);
 
 #if HAVE_BUTTON
     /* Wait some time in case the button has some debounce capacitor */

src/stm32l1/target_stm32l1.c

@@ -27,8 +27,15 @@
 #include "config.h"
 #include "backup.h"
 
+#ifndef REG_BOOT
+#define REG_BOOT BKP1
+#endif
+
+#ifndef CMD_BOOT
+#define CMD_BOOT 0x544F4F42UL
+#endif
+
 //#define CMD_FAST_BOOT 0xfa57b007
-static const uint32_t CMD_BOOT = 0x544F4F42UL;
 
 void target_clock_setup(void) {